Apparatus and method for forming a high pressure plasma discharge column

ABSTRACT

An apparatus for producing a stable, high pressure plasma column with long length, and high axial uniformity. Rotating a gas-filled tube about an horizontal axis creates a vortex with minimal, or no shear flow. Such a vortex provides a stable equilibrium for a central column of high temperature gas and plasma when, for a given rotation speed, the centrifugal force dominates over the gravitational force inside the smallest radial dimension of the containment envelope. For gas pressures sufficiently high that the particle mean free path is short compared with the thickness of the gas layer between the central plasma column and the wall, thermal transport across this sheath layer is small and its temperature is low. High pressure discharges inside a rotating envelope may be sustained by a variety of means, including electrical, electromagnetic and chemical; they may find application in plasma torches, light sources, etc. One preferred embodiment used direct current between co-rotating electrodes to sustain a one-meter-long plasma column less than 5 mm in diameter. Another preferred embodiment employed microwave heating to produce a perfectly centered plasma flame 0.5 meters long into which tens of kilowatts of power can be coupled.

This is a continuation of prior application No. PCT/US01/24376, filedAug. 3, 2001 and designating the U.S., which is a continuation of U.S.patent application Ser. No. 09/632,651, filed Aug. 4, 2001, now U.S.Pat. No. 6,417,625 which are hereby incorporated herein by reference inits entirety. The entire disclosure of the prior application, from whicha copy of the oath or declaration is supplied under paragraph 3 below,is considered as being part of the disclosure of the accompanyingapplication, and is hereby incorporated by reference therein.

The present patant document claims priority wider 35 U.S.C. § 119(a) toInternational Application No. PCT/US01/24376 filed Aug. 3, 2001, whichclaims priority under 35 U.S.C. §365(c) to U.S. patent application Ser.No. 09/632,651, filed Aug. 4, 2000, by Brooks et al., which is now U.S.Pat. No. 6,417,625 the entirety of which is hereby incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to high pressure, plasma dischargedevices. In particular, the present invention pertains to the stabilityof high pressure plasma columns within such devices conferred by themechanical rotation of their envelopes. Also, the present inventionrelates to methods and apparatuses for forming stable plasma columns athigh pressure in both open-ended and sealed discharge containmentenvelopes so as to produce plasma torches, waste reprocessing devices,ultraviolet, visible and x-ray light sources, or high intensityillumination.

BACKGROUND OF THE INVENTION

Over the years, many devices have been evolved which attempt togenerate, control and use high pressure plasmas for variousapplications. Each of these devices has certain disadvantageouslimitations. The embodiments of the present invention overcome many ofthe disadvantages of existing technologies by utilizing rotation of thecontainment envelope to establish a nearly perfect, rigid rotor flow ofthe gases within. The artificial gravity associated with this rotationalflow acts to both center and confine a bubble of plasma and hot gases.

In the context of the present patent document, a plasma is a vapor(usually created from a plasma forming fill in gaseous phase) whichincludes both neutral particles and charged particles, the latterconsisting of electrons and ions. The ions, in turn, may be acombination of atomic ions, charged radicals and/or molecular ions, inwhich the balance among these different species is dependent ontemperature, pressure and the nature of the plasma forming fill. Bycarefully controlling the environmental conditions in and around theplasma, a plasma column can be formed which is physically isolated fromthe material boundary of the confinement envelope by an interveninglayer of neutral gas. Thus, the plasma column may occupy a cylindricalvolume that is smaller than the volume enclosed by the containmentenvelope.

Since the early 60s, high pressure rf (radio frequency) heateddischarges have relied on the use of a “swirl gas” to center the plasmacolumns produced in such electrodeless devices and to prevent hot plasmafrom contacting the walls of the containment envelope. Because of thecirculatory flow of the injected swirl gases, the term “vortexstabilization” is generally used to characterize this technique. Theswirl gas method of vortex stabilization has been applied to a broadrange of rf devices. See, for example, Boulos, M. I., in “TheInductively Coupled Radio Frequency Plasma,” High Temp. MaterialProcesses, 1, 17 (1997) or Reed, T. B., J. Appl. Phys., 32, 821 (1961)both of which are hereby incorporated by reference. The swirl gastechnique makes it possible to establish plasma columns withincontainment envelopes at gas pressures from a fraction of atmosphericpressure to many times atmospheric pressure. The plasma columns need notbe straight. With the aid of vortex stabilization, a high pressure,rf-heated toroidal discharge has been achieved. See, for example, A.Okin et al., in “Generation of Toroidal Plasma—Atmospheric ArGas-Insulated Plasma Source with Quartz and Metallic Discharge Tubes,”Proc. Symp on Plasma Science for Materials, ISSN 0919-7621 (1993) whichis hereby incorporated by reference.

In addition to electrodeless discharges heated with rf power, the swirlgas method of vortex stabilization has been applied in electrodeless,high pressure torches powered with microwaves. (e.g., U.S. Pat. No.5,671,045).

Although the plasma columns formed by the swirl gas technique of vortexstabilization are approximately centered within their containmentenvelopes and stationary in a gross sense, they are not quiescent on themicroscopic level. Viscous drag on the stationary wall of thecontainment envelope results in a sheared flow of the gas whichgenerates turbulence, giving rise to enhanced thermal transport ofenergy from the hot plasma column to the cooler envelope walls. Such adegradation in the insulating properties of the annular sheath of coolergas surrounding the plasma column necessitates an increased power tosustain the plasma discharge. Additionally, turbulence causes mixing ofthe gas within the sheath and between the gas in the plasma column andthat in the sheath. As a practical matter, when such a technique ofvortex stabilization is used, the turbulence so generated manifestsitself as excess noise in the light emission of the plasma column, whichcan degrade the accuracy of certain spectroscopic measurements. As afurther practical matter, the tip of the plasma plume in vortexstabilized torches is not perfectly stable, demonstrating thecharacteristic of “wandering” or “flickering” with respect to the centeraxis of the torch. Therefore, such torches are unsuitable forapplications in which precise positioning is required.

The concept of rotating a containment envelope has been used in “sulfurbulb” lamp technologies (e.g., U.S. Pat. Nos. 4,902,935 and 5,404,076)to minimize variations in the spherical bulb's surface temperature (thatis, to eliminate hot spots caused by locally high electric fieldsoccurring in resonant microwave cavities or in coaxial terminationfixtures) and to make the spatial distribution of visible light emissionfrom these bulbs more uniform. The patents referenced above deal onlywith sealed bulbs powered by microwaves for application to highintensity lighting. To eliminate hot spots, these patents state that theaxis of rotation should be oriented in a certain angular range withrespect to the electric field direction in the resonant microwavecavity.

Therefore, what is needed is a method and apparatus for producing astable, high pressure plasma discharge that can be sustained with aminimum of power. Also, there is a need for creating stable plasmacolumns which are both long and straight. Also, there is a need formethods and apparatus for generating a stable plasma column inside acontainment envelope whereby the effects of shear flow-generatedturbulence and buoyancy-driven radial convection are substantiallyreduced in the gas outside the radius of the plasma column. There isalso a need for methods and apparatus for forming a plasma torch havinga plasma plume, or flame, which maintains a stable position centered onan axis of rotation of a containment envelope.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for forming a highpressure plasma column in a stable equilibrium.

In various embodiments a stable high pressure plasma column is producedby an apparatus comprising i) a containment envelope having aplasma-forming fill inside, ii) means for rotating the containmentenvelope, iii) means for initiating a discharge (that is, achieving“breakdown” or “ignition”) in the plasma-forming fill, and iv) means forheating and sustaining the resulting plasma in a steady state or pulsedmanner.

Ignition of the high pressure discharge inside the rotating containmentenvelope may be accomplished by means of electrical, electromagnetic(such as radio frequency waves, microwaves, or light waves) or chemicalsources of energy. These same sources of energy, individually or incombination with one another, may be used to heat and sustain theresulting plasma in a steady state or pulsed manner. The means ofignition and sustainment may utilize the same source of energy, ordifferent ones.

Various embodiments provide methods and apparatus for rotating saidenvelope so as to bring the vapors inside the containment envelope intoco-rotation with the spinning envelope. Under such conditions of rigidrotor flow, a radially outward directed, artificial gravity isestablished which forces more dense gases (colder gases) outward towardthe wall of the containment envelope and less dense vapors (plasma andneutral gases that have been heated) inward toward the axis of rotation.Buoyancy-driven radial convection of the contained vapors is suppressedin an annular region outside the cylindrical surface at which themagnitude of the artificial gravity produced by rotation just equals themagnitude of the radially directed component of earth's gravity. Theannular region in which radial convection is suppressed is hereinafterreferred to as the quiescent region. Also, turbulence caused by shearedcirculatory flow is absent in this quiescent region. The excellentinsulation properties of neutral gas in this quiescent region makes itpossible to sustain an interior discharge column with a minimum ofpower. However, the artificial gravitational force produced by rotationbecomes increasingly weak with decreasing radius, falling to zero on therotational axis. Inside the radius at which the constant downward pullof earth's gravity dominates in magnitude over the centrifugal forceinduced by rotation, buoyancy-driven radial convection cannot besuppressed, Gas inside this radius mixes and rigid rotor flow cannot besustained.

Further embodiments contemplate both sealed and open-ended containmentenvelopes. In the open-ended ones, a forced axial flow of gas may beproduced by injecting gas at positive pressure parallel to the rotationaxis; or a natural, buoyancy-driven axial flow, by inclining therotation axis of the envelope. Axial flow, forced or natural, can beused to extend the plasma column out one end of the open-ended envelopeto form a plasma plume or “flame” of a plasma torch.

Additionally, the principles illustrated by the embodiments describedherein contemplate methods and devices for making very long plasmacolumns.

Other features of the present invention are disclosed or made apparentin the section entitled “DETAILED DESCRIPTION OF THE EMBODIMENTS”.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention, reference is madeto the accompanying drawings in the following Detailed Description ofthe Embodiments. Reference numbers and letters refer to the same orequivalent parts of the invention throughout the several figures of thedrawings. In the drawings:

FIGS. 1(a) and 1(b) are perspective and end views depicting gas behaviorin a typical swirl gas torch.

FIGS. 2(a) and 2(b) are perspective and end views of gas containingenvelopes showing some effects of envelope rotation.

FIG. 3 is a schematic illustrating the generic features common to mostof the embodiments described herein.

FIG. 4(a) depicts the usual convex-upward shape of an electric arc inambient air. FIGS. 4(b), 4(c) and 4(d) are side view cross sections andan axial section view of two embodiments of the present inventionenergized by an electric current flowing through the length of theplasma column.

FIGS. 5(a-d) are a sequence of side views of an embodiment illustratinghow non-mechanical means can be employed to elongate the discharge.

FIG. 6 is a side view cross-section of the embodiment described in FIG.4(c) with adaptations for application as an intense ultraviolet lightsource, a spectral lamp of the elements and a light source for excitingunknown samples in atomic emission spectroscopy.

FIG. 7 is a side view of the embodiment described in FIG. 4(c) withadaptations for application as a plasma torch.

FIG. 8 is a side view of another plasma torch embodiment in which pulsedelectrical energy is used to raise the temperature in the plasma columntransiently into the keV regime.

FIGS. 9(a), 9(b) and 9(c) are sectional side, close-up partial sectionand A—A section views, respectively, of an embodiment powered bymicrowaves.

FIGS. 10(a), (b) and (c) are cross-sectional cuts through thecontainment envelope which reveal the different shapes of the plasmaplume produced in the absence of axial gas flow.

FIG. 11 is a cross sectional view through the containment envelope ofthe microwave-powered embodiment showing the effect of an axial gas flowon the plasma flame.

FIG. 12(a) is a cross-sectional schematic view of an embodiment using aplurality of resonant cavities to sustain an elongated plasma column.

FIGS. 13(a) and 13(b) are side and cross-sectional views of a radiofrequency-driven embodiment.

FIGS. 14(a) and 14(b) illustrate alternate means for bringing axiallyflowing gases into co-rotation with the containment envelope.

FIGS. 15(a) and 15(b) depict alternate means for creating an axial flowof gases through the envelope.

FIGS. 16(a)-(d) are perspective and side views of an embodiment of thecontainment envelope.

FIG. 17 is a side cross-sectional view of a dual-gas embodiment of thepresent invention.

FIG. 18 is a side cross sectional view of an embodiment of a diffusiveseparator based on the dual gas embodiment of FIG. 17.

FIG. 19 is a side cross sectional view of an embodiment employing amedium pressure mercury lamp.

FIG. 20 is an end view of a rotor of a air bearing of the embodiment ofFIG. 19.

FIG. 21 is a graph showing UV radiation efficiency versus current in theembodiment of FIG. 19.

FIG. 22 is a graph showing the measured cross-sectional area of theplasma column versus current in the embodiment of FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

The traditional method of vortex stabilization uses a “swirl gas” tocenter a plasma column in electrodeless gas discharges, for example,those used in plasma torches. FIGS. 1(a) and 1(b) depict gas and plasmabehavior in a typical swirl gas torch powered by rf or microwaves. Atangential flow of gas 20 is injected into an end of the envelope 10creating a vortex which confines the hot gas and plasma to a centrallypositioned plasma column 11. The swirl gas 20 forms an annular layerwhich separates the plasma column 11 from the envelope wall 14. Becausethe wall of the tube is stationary, viscous drag in the gas layerbounded by the wall causes the radial velocity distribution of the gasin the vortex to deviate from that of a rigid rotor. The rotationalvelocity assumes a peak at some intermediate location between the axisand the wall and a value near zero at the radius of the stationary wall.The strongly sheared flow pattern near the wall generates whirlpools oreddies which propagate radially inward, inducing mixing and turbulence.Because the boundary conditions for rigid rotor flow are not satisfied(due to the presence of the stationary wall of the containmentenvelope), the swirl gas method of vortex stabilization is necessarilyaccompanied by turbulence, which has the undesirable effect of enhancingradial heat transport. Furthermore, a swirl gas vortex loses strengthwith axial distance from the point of injection of the swirl gas; thissets a limitation on the length of the plasma column which may becentered by this method.

For a more complete explanation and understanding of such shear flow andturbulence effects, reference is made to A. Einstein's 1926 paper: “DieUrsache der Maeanderbildung der Fluszlaeufe und des sogennanntenBaerschen Gesetzes, Die Naturwissenshaften XX,223 (1926) which is herebyincorporated by reference.

In contrast to conventional devices, the embodiments described hereinadvantageously use a containment envelope rotating about a horizontal,or substantially horizontal axis, to create a vortex flow in which thecollective motion of the gas atoms inside closely approximates rigidrotor flow. Here, the word “horizontal” means perpendicular to thegravitational force, regardless of whether the unidirectionalgravitational force is generated by the earth's mass or by thecirculation of the containment envelope about an axis external toitself. Given sufficient time to come into equilibrium with the rotatingenvelope, the gas fill inside a sealed containment envelope must attainan angular velocity which is independent of radius, which is equivalentto saying, a tangential velocity which increases linearly with radius.Such a radial distribution of velocities is exactly that which gasparticles would have were they fixed to a rigidly rotating disk. Thevariety of advantages rigid rotor flow holds over the sheared flowpattern produced by conventional technologies will be discussed indetail below. One advantage that is immediately apparent is theavoidance of excess thermal losses caused by turbulence.

In accordance with the principles of the present embodiments andreferring to FIGS. 2(a) and 2(b), a containment envelope 10 having awall 14 is provided. The inside of the containment envelope 10 containsa plasma forming fill 12 which becomes heated and ionized into plasmawhen appropriately energized. As a consequence of the rotation of theenvelope 10 about an axis 30, a plasma column 21 is formed which isisolated from the wall 14 by a sheath 32 of neutral gases. Rigid rotorrotation of the gas and plasma inside the containment envelope obtainseverywhere outside some radius r_(c), the magnitude of which depends onthe angular velocity of rotation ω, in a manner that will be describedlater.

In a preferred embodiment, the envelope 10 is a hollow cylindrical tubecomprised of fused quartz. However, the envelope 10 can be constructedof a wide variety of other materials including, but not limited to,glasses, metals, ceramics, or composite materials. Also, a wide varietyof shapes may be incorporated into envelope design. Further, thecontainment envelope can be sealed or unsealed, the latter havingparticular utility for systems requiring an axial flow of gases. Theprecise selection of materials and envelope dimensions depends on themethod of heating and the application and may be determined by onehaving ordinary skill in the art. For example, if a microwave source isused to energize a plasma within the envelope 10, the envelope needs tobe constructed of materials transparent to microwaves. The inventorscontemplate the use of a vast array of plasma forming fills including,but not limited to, gases, liquids, vapors and even combinationsthereof. It should be noted that even solid materials can be used asplasma forming fill, if means are provided to heat them until theychange into vapor form. That means may be radiant heating of theenvelope wall by ultraviolet emissions from the internal dischargecolumn, or by external heating of the envelope.

Once introduced into the envelope 10, the fill 12 can be energized toform a plasma. Typical modes of energization include, withoutlimitation, an arc discharge between electrodes, exposure toelectromagnetic radiation or chemically induced energization.Energization of the fill 12 results in the formation inside the envelope10 of a plasma, that is, a hot vapor consisting of both neutral gas andcharged particles. The charged particles include electrons and ions, thelatter of which may be singly or multiply ionized atoms or molecules.Despite changes in temperature and density caused by energizing thegases in the envelope, pressure equilibrium is maintained and thepressure is substantially the same throughout the inside of the envelope10. For an open-ended tube atmospheric pressure prevails throughout thetube. As used in this patent, the term energization refers to themultiple steps of breaking down the fill to form a plasma (i.e.,ignition), heating of the plasma so formed, and sustaining the plasmadischarge in a steady or repetitively pulsed state.

In accordance with the Ideal Gas Law, which for conditions of fixedpressure states that density and temperature vary inversely with oneanother, the density of hot gas is lower than that of cooler gas. Theprinciples of the present invention take advantage of this difference indensity. Rotation of the envelope 10 causes the gases inside theenvelope to separate according to density, with the hot, less densegases collecting in the region surrounding the axis and the cooler, moredense gases moving radially outward to occupy the region near the wall.The central region of hot gases (and plasma, as is explained further onin this paragraph) is referred to herein as a plasma column 21; theregion of cooler gases lying between the plasma column 21 and the wall14 is referred to herein as an annular sheath 32. The low density of thehot gases and plasma in the plasma column 21 is figuratively representedin FIG. 2(a) by a paucity of dots; the high density of the cooler gasesin the annular sheath 32 by an abundance of dots. Since the ionizationfraction of a gas under the collisional conditions of high pressure iscontrolled by temperature, the region occupied by plasma is effectivelyone and the same as that occupied by the hot gas.

With continued reference to FIGS. 2(a) and 2(b), the plasma column 21 ofhot gas and plasma produced upon energization of the plasma-forming fillbehaves just as air bubble does in water, that is, it has a tendency torise. The bubble of hot underdense gaseous fluid collected in thecentral column experiences a buoyancy force associated with thedifference in specific gravity of the bubble compared with that in thesheath. But whereas the buoyancy force is independent of position, thecentrifugal force increases with distance from the axis of rotation.Therefore, outside some critical radius the value of which depends onthe rotation frequency, the centrifugal force dominates over thebuoyancy force. So long as the plasma-forming fill may be represented asa gaseous fluid such that the variation of specific gravity (i.e., massdensity) with radius is only a function of temperature, buoyancy-drivenconvection is eliminated outside of the critical radius; inside thatradius, thermal transport by convection may take place. Thus, rotationof the containment envelope about a horizontal axis creates a stableequilibrium for a central plasma column, so long as the mass density ofthe energized fill increases with radius from the rotation axis.

In a mathematical representation, the centrifugal force per unit volumeis defined as ρ r ω² where ρ is the specific density of the gaseousfluid, r is the distance from the axis of rotation 30 and ω is theangular rotation frequency of the envelope. The buoyancy force per unitvolume is defined as ρ g, where ρ, again, is the specific density and gis the gravitational acceleration The critical radius r_(c) at which thecentrifugal force balances the buoyancy force is independent of ρ andgiven simply by r_(c)=g/ω². For a rotation speed of 1400 rpm, theformula above gives a value for r_(c) of just 0.5 millimeters. Insider_(c) the buoyancy force dominates over the centrifugal force, outsidethe reverse is true. Thus, rotation of the envelope is effective insuppressing buoyancy-driven radial convective heat transport inside theenvelope only when ω>(g/R)^(1/2), where R is a characteristic radialdimension of the envelope 10.

The formula for the critical radius given above can only be approximatebecause radial convective transport inside the critical radius precludesthe possibility that rigid rotor flow obtains within this radius. Thus,a mismatch must exist in the rotational velocity of the inside andoutside of the interface defined by the formula for the critical radiusgiven above. Eddies generated by this mismatch may blur the interface,leading to an effective critical radius somewhat larger than that givenby the simple formula above. Shear flow-generated turbulence andbuoyancy-driven radial convection are suppressed in the annular sheath32; in the plasma column 21, they are not. However, in the absence ofhigh axial flow velocities associated with injected streams of gas, theReynolds number for the fluid flow in the central column should be muchlower than that in the case with the swirl gas method of vortexstabilization. If laminar flow conditions are maintained, turbulencewithin the central plasma column may be avoided.

The existence of an annular sheath 32 of colder gas surrounding theplasma column 21 is an important feature for the present embodiment.However, this is possible only when gas pressure is “high.” In thiscontext, high gas pressure is any pressure greater than that for whichthe average distance traveled by a gas or plasma particle betweensuccessive collisions is short compared with the thickness of theannular sheath 32. This condition is equivalent to saying that the gasin the sheath behaves like a fluid in which hydrodynamic flow obtainsThe average distance between collisions is generally referred to as themean free path, λ. The density within the containment envelope, n, andthe cross section for interaction, σ, by the relation λ=1/n σ. Since σhas only a weak dependence on temperature, variations in λ withoperating conditions are essentially determined by density alone. Inaccordance with the principles of the present invention, the minimumpressure required for the existence of an annular sheath depends on therotational velocity of the envelope and the radius of that envelope,since the thickness of the sheath is set by the difference between theradius of the containment envelope and the critical radius for a givenrotation speed. For example, at standard conditions of temperature anddensity (STD) the mean free path in argon is about four microns.Therefore, a sheath with a thickness ten times this value would sufficeto “float” a central column of hot gas and plasma within the envelopeand thermally isolate the column from the wall of the containmentenvelope.

An important advantage realized by the ohmically heated embodimentsdescribed herein is that the radius of the plasma column 21 may be madevery small. With applied electric field, energy dissipation within theplasma column tends to be greatest where the temperature is highest.This results in a feedback mechanism whereby regions already hot, gethotter. A lower limit on the minimum achievable diameter of the plasmacolumn is given by the critical radius r_(c), because buoyancy-drivenconvection flattens the temperature profile inside this radius.

The separation of the discharge into a central plasma column 21surrounded by an annular sheath 32 requires that the gas in the centralcolumn, as well as the gas in the sheath, behave like a fluid. The meanfree path inside the plasma column 21 is more than an order of magnitudegreater than that in the sheath, because of the low density of theheated gas and plasma on axis. For a temperature of 1 eV and atmosphericpressure, the mean free path in argon is roughly 150 microns; for apressure 1/100th of an atmosphere, the mean free path is 15 millimeters.The condition that the mean free path for a particle on axis be smallcompared with the diameter of the plasma column sets a lower limit onthe pressure for which the embodiment discussed here is viable within acontainment envelope of a given diameter.

While the above embodiments are described as employing an axis ofrotation that is horizontal, it is also contemplated that otherembodiments will employ an axis of rotation that is off-horizontal.Elevating the axis of rotation off horizontal has the effect of creatinga natural convection in the axial direction of the heated gases in theplasma column. With increasing elevation angle, the axial flow becomesstronger and more sheared in the radial direction. Rotation of thecontainment envelope makes it possible, nevertheless, to maintain anannular sheath of neutral atoms which both center the plasma column andprovide thermal and electrical isolation of it from the wall of thecontainment envelope, Because the axial flow of heated gas entrains andtransports the plasma, the embodiment with a rotation axis offhorizontal may be described as a rotating plasma chimney. When theReynolds Number for the convective flow velocity in the axial directionexceeds the threshold for onset of turbulence, laminar flow will nolonger obtain and the thermally insulating property of the annularsheath will begin to degrade.

FIG. 3 is a schematic block diagram which illustrates the genericfeatures common to the embodiments described herein. Included are i) acontainment envelope 10 which contains therein a plasma-forming fill 81,ii) means 70 for rotating the containment envelope 10 about ahorizontal, or substantially horizontal axis 30, iii) means 73 forinitiating a discharge, that is, achieving “breakdown” or “ignition,” insaid plasma-forming fill, and iv) means 74 for heating and sustainingthe resulting plasma in a steady state or pulsed manner. Furthermore,the rotational frequency of the envelope 10 must be sufficiently highthat the plasma induced by energizing means 73 and 74 is confined insidea plasma column 21, the surface of which does not intersect the wall ofthe containment envelope. An exception to the class of embodimentsrepresented in FIG. 3 is the embodiment described earlier as a rotatingplasma chimney, in which the axis of rotation is elevated with respectto the horizontal.

As discussed previously the embodiments can include containmentenvelopes 10 with a variety of sizes, shapes and materials chosen inaccordance with the needs of the user. Satisfactory envelope materialsinclude, without limitation, glass, metals, ceramics, or compositematerials. The size and shape of the containment envelopes may encompassa wide variety of shapes, e.g., open-ended tubes, sealed bulbs (such asthose used in lamps), or irregular shapes (such as the one shown in FIG.9(a)). The only real limitation on the size of the containment envelopeis that the minimum dimension perpendicular to the rotation axis belarger than the radius of the plasma column 21 contained therein. Theplasma-forming fill 12 can comprise nearly any ionizable material, withthe particular needs of the system dictating the precise choice ofmaterial. The fill can be a gaseous, liquid or solid material, with themass of the latter two preferably small enough that the energy input tothe discharge is sufficient to convert that material into vapor form.Preferred gases include air, xenon and argon. Additionally, theplasma-forming fill can include more than one material, for example,prepared gas mixtures used in lasing media, waste materials, and thecombination of rare gases with optical dopants like lithium, sodium andsulfur.

The envelope rotation system 70 may include a number of means forrotating an envelope 10 as known to those having ordinary skill in theart. One preferred means for rotating the envelope 10 includes avariable speed electrical drive motor operatively connected to theenvelope 10, for example, using a drive belt. A large variety of otherrotating means are also contemplated by the inventors as being withinthe scope of the invention.

Energization of the plasma may be accomplished by a variety of methods,including without limitation, electrical arc discharge betweenelectrodes, chemical reaction between reactive components containedwithin the envelope, or discharge induced by the use of electromagneticradiation, such as microwaves, radio frequency waves, light waves, orother methods known to those having ordinary skill in the art. As statedearlier, the term energization encompasses both the means 73 forigniting a discharge in the plasma-forming fill and the means 74 forheating the resulting plasma and sustaining it in a steady state orpulsed manner. Different methods, e.g., electrical and electromagneticradiation, may be combined in a single embodiment, with one methodpreferably used for ignition and the other method for heating andsustaining the plasma column, or one method used for ignition and acombination of methods used for heating and sustaining theplasma-column.

FIG. 4(a) illustrates an ordinary arc discharge 41 between a pair ofelectrodes 42 in ambient air. To create such a discharge, an electricalpotential is applied between the pair of electrodes 42 with an highvoltage power source 43. The luminous path of such an arc discharge 41is convex upward in shape, as suggested by the name “arc.” By rotatingthe containment envelope 10 as previously described with respect to theembodiment shown in FIGS. 2 and 3, arc discharges 41 are produced whichare virtually straight lines. Rotating the containment envelope confersthe further advantage that the straight arcs therein may be made quitelong, for example, the gap X between the electrodes 42 may be severalmeters long.

One particular embodiment is illustrated in FIG. 4(b). A cylindricalenvelope 10 is provided which, by way of example, may be formed of afused quartz cylinder having an inside diameter of 48 mm, a wallthickness of 2 mm and length of 1.2 m. Two electrodes 42, a firstelectrode 42 a and a second electrode 42 b, are mounted along thehorizontal rotation axis 30 from opposite ends of the containmentenvelope 10, with one or both electrodes configured to retract axiallywhile co-rotating with the envelope. In the preferred embodiment shownin FIG. 4(b) the second electrode 42 b is configured to retract axiallyin the direction given by the arrow A, whereas the first electrode 42 ais fixed in the axial direction. The electrodes 42 are preferablyconstructed of a refractory metal such as tungsten (W) with a diameterof about 1.5 mm and a length of about 50 mm.

In order to generate an electrical potential between them, theelectrodes 42 a and 42 b are electrically connected to a power source 43by means of electrical brushes 45 which contact the electricallyconducting end caps 44. These end caps co-rotate with the envelope andprovide support for the fixed electrode 42 a and the retractableelectrode 42 b, the latter by means of the sliding arm 46 to whichelectrode 42 b is mounted. The envelope 10 is purged with a gas fill 12,preferably a noble gas like argon or xenon. The purge gas is fed througha gas feed tube 47 at slight positive pressure into the space 49enclosed between the end cap 44 and the cup 48, the latter making asliding seal against the end cap 44. Passage through the holes 50 in theco-rotating end cap 44 brings the purge gas into co-rotation with thecontainment envelope 10. Alternatively, if the end caps make gas-tightseals to it, the envelope may be charged with a gas fill 12 at pressuresranging from sub-atmospheric to super-atmospheric. The envelope issupported, for example, on a pair of bearings 66 which allow rotation ofthe envelope 10 by a rotating means 70 comprised, for example, of amotor 71 and a belt 72.

With the points of the electrodes close together or touching, highvoltage is used to ignite an arc between them. In the embodimentdepicted in FIG. 4(a), a current-regulated direct current power source43 with a maximum output voltage of 6 kV is sufficient to ignite an arcif the electrodes are less than 5 mm apart. The magnitude of theelectrical potential required to sustain the discharge is determined bya variety of factors including the nature of the plasma forming fill 12,the pressure of the fill, the rotational frequency of the containmentenvelope 10, the magnitude of the arc current, and the distance betweenthe electrodes following their separation. One example of the embodimentpictured in FIG. 4(a) uses an argon fill 12 at atmospheric pressure, arotational frequency of 3,000 rpm, an arc current of 1 amp, and a finalseparation between electrodes of 1 m. Only 500 volts is required tosustain this discharge. The luminous diameter of the plasma columnproduced in this manner is less than 5 mm.

Other examples of the embodiment have utilized 60 Hertz alternatingcurrent (supplied from high voltage transformers with magnetic shunts)both to ignite and to sustain the arc. Because the arc plasma nearlyextinguishes at each zero crossing of the alternating current waveform,a significant fraction of the available power during each half cycle isutilized to reheat the plasma.

In the embodiment represented by FIG. 4(b) the tips of the electrodes 42are immersed in the hot plasma of the central plasma column 21 and,consequently, subject to melting, vaporization and erosion when the arccurrent is high. In applications which require generating a vapor richin one of the constituent elements of the electrode, locating theelectrodes on axis and, thereby, exposing the electrodes to high heatload can be advantageous. If, on the other hand, elements from theelectrode constitute undesired contamination of the fill gas 12, theelectrodes 42 may be located outside the radius of the plasma column 21,as illustrated in the embodiment represented by FIG. 4(c).Alternatively, it may be desirable in some applications to locate oneelectrode on the axis 30 and to locate the other outside the radius ofthe plasma column 21.

With continued reference to FIG. 4(c), each electrode is in the form ofa ring 142 with an opening 143 larger than the outside diameter of theplasma column 21. Alternatively, each electrode may be a single straightrod positioned off axis. As taught in the embodiment illustrated in FIG.4(a), the ring electrodes 142 are preferably fabricated of a refractorymaterial such as tungsten and supported off the end caps 44 at oppositeends of the envelope 10. To draw an arc between the axially separatedelectrodes, a co-rotating, electrically conducting rod or tube 51, whichinitially bridges the two electrodes, is pulled out one end of theenvelope. As the striking rod 51 is removed, the arc spanning the gap Xbetween it and the first ring electrode 142 lengthens. Once the arcbridges both electrodes 142, the striking rod 51 can be fully removedfrom the envelope 10. Alternatively, the electrodes may be made axiallyretractable, as illustrated in FIG. 4(b), and an arc may be struck withthe two electrodes close together or touching. In the embodimentillustrated in FIG. 4(c) but not shown, the envelope is supported onbearings which allow rotation of the envelope 10 by some rotating meansand the electrodes are electrically connected to a power source by meansof electrical brushes contacting the end caps.

FIG. 4(d) is a close up view of the circled portion in FIG. 4(c)containing the plasma column 21 and one ring electrode 142. Thedischarge path in the vicinity of each ring electrode 142 consists oftwo distinctly different plasma regions: the central plasma column 21and a radial segment 35 lying between the electrode ring 142 and thesurface of the plasma column 21. Because the gas outside the centralplasma column is cold and therefore electrically resistive, the voltagedrop per unit distance is much higher across the radial segment 35 ofthe discharge path than along the axial plasma column 21. The gasesheated by the discharge in the radial segment 35 are driven toward thecentral column by the artificial gravity created by rotation of theenvelope, as taught earlier in this patent. This inward convectioncreates a dipole-like convective flow 85 which cools the surface of thering electrode 142. Since the ring electrode 142 co-rotates with theenvelope, the pattern of convective flow 85 is stationary in therotating frame of the envelope. The cooling effect of the convectiveflow 85 reduces the temperature rise of the electrodes 142 compared withthat of axial electrodes (see FIG. 4(b)) immersed in hot plasma. As aresult, electrode life is extended. However, this natural cooling doesnot eliminate sputtering processes by ions accelerated by theelectrostatic potential across the radial gap between the ringelectrodes 142 and the plasma column 21.

In the embodiments illustrated in FIGS. 4(b) and 4(c), ignition of thedischarge is achieved with the tips of the electrodes close together;the resulting arc is then elongated by mechanically retracting aco-rotating electrode (FIG. 4(b)) or withdrawing a co-rotating rod (FIG.4(c)). An alternative embodiment is illustrated in FIG. 5, which permitselongating the arc without mechanical movement of any component insidethe discharge tube. The right electrode 42 is electrically extended inthe direction of the left electrode 42 with a wire 42 c lying againstthe wall of the containment envelope 10. With application of asufficiently high voltage an arc 35 is struck across the gap between theleft electrode 42 and the tip of wire 42 c. Since the discharge requiresconsiderably lower voltage to sustain than to ignite, the cost of thepower supply system may be minimized by using a fast-opening switch 43 sin parallel with a low voltage, de power supply 43 series-connected to alarge inductor 43 i.

Elongation of the arc is achieved by creating a convective flow 13 ofgas in the axial direction. The flow 13 may be driven by an externalsource of gas at positive pressure or by tilting the rotation axis 30 ofthe containment envelope 10 off horizontal (neither of which means forinducing said flow 13 is shown in FIG. 5). Forced or natural convectionof gas in the axial direction causes the electric arc 35 to bow out inthe direction of the flow 13. The plasma and heated gas in the portionof the arc closest to the axial electrode will be forced toward the axis30 of the rotating containment envelope 10, while the point of contactof the arc with wire 42 c will travel in the direction of the convectiveflow. FIGS. 5(a-c) illustrate in snapshot fashion the manner in whichthe axial plasma column 21 elongates to the right until it contacts theright electrode 42 in FIG. 5(c). In its traversal down the length of thecontainment envelope under the influence of convective flow 13, themovement of the L-shaped arc in the rotating containment enveloperesembles, except for the matter of direction, the behavior of an arc ina Jacob's Ladder.

Embodiments with hollow electrodes may be used as light sources withvarious purposes: for example, intense ultraviolet light sources, brightspectral emission lamps for use in analytic chemistry, light sources foratomizing or exciting unknown samples in atomic emission spectroscopy,and sources of general illumination.

FIG. 6 illustrates one light source embodiment 600 which includes acontainment envelope 10, sealed at opposite ends by a first and a secondend cap 644 a, 644 b and having therein a pair of ring electrodes 142and a plasma column 21. The light source 600 can be electrically poweredand connected in a manner discussed with respect to FIGS. 4(b) and 4(c).The plasma column 21 produces light, including a light beam 601 which isprojected through a light transmissive window 602 in the first end cap644 a positioned at a first end of the envelope 10. The second end cap644 b, positioned at an opposite second end of the envelope 10, caninclude a pump-out fitting 643 which permits evacuation of thecontainment envelope 10 and subsequent backfill with a gaseousplasma-forming fill 12 at sub-atmospheric or super-atmospheric pressure.An arc may be initiated between the electrodes 142 using, for example, astriking rod 651 (shown in retracted position). The striking rod must betranslated with means external to the containment tube, such as amagnetic actuator, to transfer the arc to the electrode 142 attached tothe second end cap 644 b. Once a discharge has been ignited, thestriking rod must be retracted from between the two electrodes 142. Aparticular advantage of the present light source 600 is the axialtransmission of a light beam 601 from a long column of emitting atoms orions.

In other sealed light source embodiments it is advantageous to use aheavy, non-reactive gas such as xenon as the fill gas 12. Xenon isadvantageous, both because of its high atomic weight (131 Atomic MassUnits (AMU)) and its low thermal conductivity (one fifth that of air).If the vapor of elements of atomic weight less than 131 AMU are presentin small quantities inside a rotating envelope 10 containing xenon, thepeaked temperature profile within the axial discharge column will resultin an enrichment on axis in the concentration of the light elements anda reduction in their concentration near the wall. Examples of usefuldopants with lower atomic mass than that of xenon include, but are notlimited to, popular spectroscopic elements like sodium, iron, nickel,copper, and cadmium. Enrichment within the plasma column 21 of elementslighter than the primary fill gas 12 means that the spectral intensityof these light elements in a light source embodiment will be enhancedover other light sources in which sample elements rapidly convect intoand out of the excitation zone.

If a dopant, upon vaporization, causes the pressure inside a sealedenvelope to rise many orders of magnitude such that the dopant becomesthe principal element comprising the fill, the presence of xenon has nomaterial effect on the relative radial concentration of the dopant.Nevertheless, a low pressure fill of a noble gas like xenon or argon canbe used advantageously to facilitate breakdown at low gap voltage in alamp, such as a medium pressure mercury lamp, in which the metal, oncevaporized, become the dominant component of the fill. Once the mercurycharge in such a lamp is vaporized by joule heating, a mantle ofneutrals atoms surrounding the axial plasma column will reduce theconducted heat load to the wall. Reduced heat load to the wall of thelamp in the present invention mitigates the need for active cooling ofthe envelope.

The embodiment of FIG. 7 illustrates a plasma torch 700 constructed inaccordance with the principles previously disclosed herein. The plasmatorch 700 includes many of the same elements as the embodiment depictedin FIG. 4(c). As in FIG. 4(c) but not shown here, an electricalpotential is applied across the ring electrodes 142 to form acurrent-carrying plasma column 21 and the envelope 10 is rotated about ahorizontal, or substantially horizontal rotation axis 30. Additionally,a convective flow 13 is used to produce a non-current-carrying plasmaflame. One way of achieving axial gas flow is by introducing a gaseousplasma forming fill 12 at positive pressure through a first end cap 44a. To bring the flowing gas into co-rotation with the envelope it isdesirable to enhance the momentum transfer to the gas over that whichwould occur solely by interaction of the gas with the walls of thecontainment envelope 10. In the preferred embodiment shown in FIG. 6auxiliary means for inducing rigid rotor flow are provided in the formof a feed space 49 defined by an outer end cap 48 and a plurality ofcollimating holes 50 in the first end cap 44 a. Another auxiliary meansfor enhancing gas rotation can also be a “honeycomb” collimator, orother devices known to those having ordinary skill in the art.

The plasma torch embodiment 700 may be employed in all the usualapplications for which such torches are used, for example, flame spraydeposition of metals, dielectrics, ceramics, metal matrixes. Otheruseful applications are discussed in the previously referenced articleauthored by M. I. Boulos. Advantageously, the small flame diameter andprecise positioning possible with the torch embodiment 700 make itattractive for cutting and drilling applications.

With reference to FIG. 8 another particularly advantageous embodimentincludes a power source 43 p capable of supplying pulsed electricalenergy to the pictured embodiment. In a preferred embodiment, the pulsedpower source 43 p is connected in parallel with the dc power supply 43,with suitable diode circuitry to protect the dc supply against damagefrom excessive voltage. For example, a capacitor bank or transmissionline can be used as a pulsed power source 43 p. Following formation of aplasma column with the continuous dc supply 43, the pulsed power source143 is discharged through a fast-closing switch S, causing an highcurrent to flow through the plasma column 21. If the current rises sofast that it does not have time to “soak in” radially, a skin currentwill be produced on the surface of the plasma column with an associatedmagnetic field that pinches the annular current distribution toward therotation axis. Such magnetic compression can easily generatetemperatures in the 5-100 keV region, sufficient to excite Kline, X-rayemission from the elements confined in the plasma column 21. The plasmacolumn 21 can also be transiently heated using a high power, pulsedlaser. Other modes of heating are discussed in the Handbook of VacuumArc Science and Technology; Ed. by R. I. Boxman, P. J. Martin, and D. M.Sanders (1995), especially the Chapter entitled “Pulsed PowerApplication” the entirety of which is hereby incorporated by reference.

Other preferred embodiments utilize electrodeless methods for energizingthe discharge. One preferred embodiment 900 is illustrated in FIG. 9(a).As with the previously discussed embodiments, embodiment 900 includes acontainment envelope 10 containing therein a plasma forming fill 12. Theenvelope is rotated about a horizontal, or substantially horizontalrotation axis 30 by a rotation system 70, the nature of which has beenwell described herein. The pictured embodiment further includes amicrowave source 60 as a means for both initiating and sustaining thedischarge. A satisfactory microwave source is, for example, a 2.45 GHzcontinuous wave, 30 kW magneton power supply such as that manufacturedby Microdry Corp. Other examples include microwave sources at ISM(Industrial Scientific Medical) standard frequencies, including but notlimited to 896 MHZ and 950 MHZ. The latter frequencies are advantageous,because high power magnetron sources operating at these frequencies canbe obtained inexpensively from commercial vendors. In the picturedembodiment the envelope 10 is supported, for example, on a pair ofbearings 66 which allow rotation of the envelope 10 by a rotation system70 comprised, for example, of a variable speed AC motor 71 and a drivebelt 72.

Microwaves generated by the power source 60 are introduced into ashorted section of waveguide 61 (e.g., a WR430 waveguide) so as toproduce a standing wave pattern with an effective wavelength betweennodes of λ_(eff), measured along the axis of the waveguide 61. Therotation axis 30 of the envelope 10 is positioned transverse to the axisof the waveguide 61, at the location of an anti-node in the standingwave pattern. In the embodiment depicted in FIG. 9(a) the rotation axis30 is positioned at a distance of ¾ λ_(eff) from the shorted end 62.Openings 63 in the broad-sided wall of the rectangular waveguide 61permit the envelope 10 to pass through.

In another aspect of the embodiment 900 illustrated in FIG. 9(a),electrically conductive sleeves 67 surround the envelope 10. Thesesleeves 67 serve as shields against radiation leakage out of theopenings 63 in the waveguide 61. A preferred embodiment includesaluminum sleeves 67 having an air gap less than two mm (millimeters)wide between the envelope 10 and the encircling sleeve 67. Rotation ofthe envelope 10 inside close-fitting stationary sleeves 67 leads toefficient heat transfer between the envelope 10 and the sleeves.

A further aspect of the embodiment shown in FIG. 9(a) includes aresonant coaxial cavity 200 attached to the left side of waveguide 61.The resonant coaxial cavity 200 serves as a means for igniting a plasmain the plasma forming fill 12. This is illustrated in greater detail inFIG. 9(b). Other satisfactory means for plasma ignition are known tothose practiced in the art of rf and microwave discharges.

Referring to FIG. 9(b) a resonant coaxial cavity 200 is shown. Thecavity 200 includes an outer conducting shell 201 (comprised of elements67, 66, 68, 69) and a center conductor 93. The outer conducting shell201, which extends from the shorted wave guide 61 to an end piece 69,has a length 202 equal to an integral number N of vacuum halfwavelengths λ_(eff)/2 of the microwave radiation (in this case, 2.45GHz). The outer conducting shell, in a preferred embodiment, includes aconducting sleeve 67, a bearing ring 66, and a conducting extensionpiece 68 terminating in a conducting end piece 69. A satisfactoryembodiment includes an aluminum sleeve 67 with a cylindrical extensionpiece 68 formed from copper screen wrapped around the bearing ring 66with a brass disk forming the end piece 69. The end piece 69 includes anopening 93 therein surrounded by close-fitting rf finger joint materialenabling a center conductor 91 to be completely retracted withnegligible leakage of microwave radiation. With continued reference toFIG. 9(a) a satisfactory center conductor 91 comprises, for example, astainless steel weld rod three mm in diameter having a pointed tip atone end, and a thermally and electrically insulated handle 92 at theother end. The handle 92 facilitates convenient removal of the centerconductor 91 from the outer conducting shell 201 once the fill gas 12has been ignited.

In this embodiment plasma ignition is achieved by turning on themicrowave power supply 60 at a power setting of several kilowatts, andthen withdrawing the tip of the center conductor 91 past the side wallof the waveguide 61. Generally, a popping sound is audible when theplasma forming fill 12 breaks down (i.e., forming plasma). Once ignited,the discharge may be sustained with microwave power of less than onekilowatt.

The center conductor 91 is withdrawn completely after ignition, toprevent it from heating up excessively. Without the center conductor 91present, the outer conducting shell 201 no longer encloses a resonantmicrowave cavity and microwave energy cannot propagate in coaxial modedown the sleeve 67 any farther than the plasma column 21 extends intothe sleeve.

FIG. 9(c) is an enlarged cross-sectional view of FIG. 9(a) along cutA—A. The envelope 10 passes through the waveguide 61 at openings 63 inits side walls, The bearings 66 (not shown) hold the rotating envelope10 in position. The double-sided arrow 115 indicates the direction ofthe oscillating electric field associated with the TE₁₀ fundamental modefor transmission of the transverse electric wave inside the waveguide.

After ignition, the plasma column 21 in embodiment 900 takes one of thethree elongated shapes illustrated in FIGS. 10(a)-10(c). For example,the shape of the plasma column 21 may be that of a single-ended flamepointing to the right (FIG. 10(a)), a single-end flame pointing to theleft (FIG. 10(b)), or a double-ended flame pointing to both the rightand left of the waveguide 61 (FIG. 10(c)). For the single-ended flames(FIGS. 10(a) and 10(b)), the plasma column is described by a surface ofrevolution with a blunt end or root 21R located in the center of thewaveguide 61. From a diameter of less than 2 cm at its root 21R, theplasma column expands with axial distance to fill, or nearly fill theenvelope 10 before tapering down to a point about 5-10 cm from the wallof the waveguide 61. The maximum diameter of the plasma column liesabout 3 cm from the wall of the waveguide 61. The fact that the plasmacolumn 21 is brightest where its diameter is greatest (inside thealuminum sleeve 67) suggests that the microwave energy couples to theplasma column 21 predominately in the sleeve 67, not in the waveguide61. For this to occur, the microwaves must mode-convert from thetransverse electric wave inside the waveguide 61 to a coaxial waveinside the aluminum sleeve 67. Presumably, the plasma column 21 itselfserves as the central conductor for coaxial wave transmission along theaxial direction inside the aluminum sleeve 67. The appearance of thedischarge column may be altered by introducing a flow of gas along theaxis of the envelope as will be described later.

FIG. 11 illustrates a plasma torch embodiment 1100 of the electrodeless,microwave discharge introduced in FIGS. 9(a)-9(c). In this embodiment, afill gas 12 introduced at positive pressure in the left end of theenvelope 10 through a gas feed tube 101 is brought into co-rotation withthe envelope 10 by an aluminum honeycomb collimator 102. The collimator102 is just one of many possible auxiliary means for impartingrotational momentum to an axial gas stream. If the plasma column 21takes the form of a single-ended flame rooted in the waveguide 61pointing to the left (see FIG. 10(b)), initiation of an axial flow 13will push the plasma in the direction of the flow. As a consequence, theplasma column 21 will take the form of a flame pointing to the right,with its blunt end still rooted in the waveguide 61. Furthermore, thepresence of flow elongates the flame and gives it a more uniformbrightness along the length of the aluminum sleeve 67. Microwave energyis probably transmitted farther down the aluminum with flow thanwithout, because the extended plasma column 21 inside the aluminumsleeve 67 serves as a center conductor for coaxial mode propagation. Onesurprising advantage of such embodiments is that no upper limit (withinthe 30 kW capability of the power source 60) has been found on theamount of power which may be coupled to the plasma column 21. At highpower with suitable flow rates, the flame tip extends beyond the end ofthe envelope 10. Measurements on a burn target placed near thedownstream end of the quartz tube show that the flame tip is extremelywell centered.

The embodiment 1100 illustrated in FIG. 11 has very attractive featuresfor application as a plasma torch. First, very high microwave power maybe coupled into the plasma column 21. Second, the plasma substantiallyfills the envelope 10 in the region where microwave energy is moststrongly coupled to the column 21. The first feature gives thisinvention the capability to atomize waste streams with high throughput;the second insures that no chemicals will leak past the heating zonewithout being destroyed.

In yet another embodiment powered by microwaves, a plasma column 21 maybe formed that is both long and straight. FIG. 12 illustrates anembodiment in which a plurality of heating zones 112 repeat in modularform along the length of the envelope 10. This plurality of heatingzones makes it possible to form a plasma column 21 many times longerthan that which can be sustained using a single heating zone (See, forexample the embodiment of FIG. 10(a)). A microwave power source (notshown) is operatively connected to a shorted waveguide 110 which, inturn, feeds the microwaves 60A into a plurality of resonant microwavecavities 111 through slots 116 located at anti-nodes in the standingwave pattern of the shorted waveguide 110. Said slots 116 may beseparated from one another by a constant distance 121 equal to anyintegral number of effective wavelengths λ_(eff) for the TE₁₀fundamental mode in the input waveguide 110. Each resonant cavity 111includes openings to enable a containment envelope 10 substantiallyparallel to an axis of the input waveguide 110 to pass through theplurality of resonant cavities 111. Electrically conducting sleeves 67span the gaps between the plurality of resonant cavities 111 and serveas shields against microwave leakage out the openings in the sides ofthe resonant cavities 111. The rotational axis 30 of the envelope 10 ispositioned at the location of peak field intensity within the microwavecavities 111. As with the previously discussed embodiments, a means (notshown) for rotating the envelope 10 is provided. The resonant cavity 111closest to the shorted end 110A of the input waveguide 110 is positioneda distance 122 equal to an integral number of quarter wavelengths fromthe shorted end 110A, one preferred distance 122 being ¾ λ_(eff). Theplasma-forming fill 12 is energized by microwaves in the plurality ofheating zones 112 defined by the resonant microwave cavities 111.Because, as taught earlier in this patent, microwaves may propagate incoaxial mode inside the conducting sleeves 67, a single continuousplasma column 21 may be formed inside the rotating envelope 10. Theaxial uniformity of that plasma column 21 may be enhanced with gas flowalong the length of the containment tube 10.

Radio frequency radiation can provide another means for energizingplasma in the embodiments of the present invention. Reference to FIGS.13(a) and 13(b) illustrates a radio-frequency (rf) embodiment of thepresent invention that utilizes an induction coil 120 surrounding therotating envelope 10 to energize a plasma forming fill 12 into a plasmacolumn 21. As with conventional rf torches, satisfactory power suppliesrange in frequency from the kHz range to the MHz range. As with thepreviously discussed embodiments, a means of rotation must be providedsuch as that discussed with respect to FIG. 3.

FIGS. 14(a) and 14(b) illustrate additional means for imparting angularmomentum to the gases in an envelope 10, so as to maintain rigid rotorflow in the presence of natural or forced axial flows. As explainedearlier with respect to FIG. 4(b) and FIG. 7, co-rotation may be inducedby flowing gas, respectively, through openings in an end cap and througha honeycomb collimator disk within the envelope. FIG. 14(a) illustratesthe use of a containment envelope 10 with large length-to-diameter ratioto induce co-rotation of the flowing gas. FIG. 14(b) shows the use of anaxial feed tube 174 co-rotating with the envelope 10 to accomplish thesame result. In the latter case, a stationary gas feed tube 47 makes asliding seal against the rotating end cap 44. Other means, known tothose having ordinary skill in the art, may be used to provide rigidrotor flow of the gases inside the containment envelope 10.

FIGS. 15(a) and 15(b) shows means for inducing an axial gas flow 13through the rotating envelope 10 apart from the application of positivepressure at an inlet end depicted in FIGS. 4(b), 4(c) and 4(d). As shownin FIG. 15(a), a suction tube 81 may be inserted in the outlet end ofenvelope 10. Alternatively, as indicated in FIG. 15(b), the envelope 10can be tilted at some angle θ with respect to the horizontal, so thatthe buoyancy of the heated gas in the plasma column naturally drives anaxial convective flow 13. The axis of rotation 30 remains substantiallyhorizontal, since only a small tilt angle θ is required to induce astrong natural convection.

FIGS. 16(a) and 16(b) show an axially non-uniform containment envelope1610 which permits utilization of liquid and solid materials as part ofthe plasma-forming fill 12 in the various embodiments of a rotatingdischarge tube described heretofore. As shown in FIG. 16(a) the axiallynon-uniform containment envelope 1610 includes a bulbous portion 1611with two substantially cylindrical sections 1612 and 1613 extending outits opposing sides. As depicted in the FIG. 16(b), the bulbous portion1611 serves as a reservoir for liquid or solid material 1681 which mayby vaporized by external heating or by energy supplied from theenergizing source of the discharge (and released from the plasma column,or a portion of the plasma column 21, extending into the bulbous portion1611). The liquid or solid material 1681 undergoing phase change into avapor may constitute the sole plasma-forming fill 12 or may be used inconjunction with a gaseous fill, such as argon or xenon, that mayfacilitate ignition of a discharge.

When used in conjunction with a gaseous fill, the concentration of avapor of a liquid or solid material 1681 may be enhanced on the axis ofthe rotating containment envelope 1610, providing its vapor has a lowermass density than the chosen gaseous fill (as discussed previously).Examples of liquid and solid materials 1681 that take solid or liquidform at room temperature and may be confined in a vapor mixture withargon, include low density materials such as lithium, silicon, silicondioxide, and sulfur. Heavier materials can be used in combination with acorrespondingly denser gas, such as xenon. Medium-Z metals and theircommon salts may be confined about the axis of rotation by xenon,because of its high atomic weight of 131 AMU.

The principles described herein permit embodiments in which multiple gasconstituents may be introduced into a rotating containment envelope inthe form of concentric streams. As the axially flowing gases in theconcentric streams transit the open-ended containment envelope theatomic and molecular constituents of the separate streams mix only bythe slow process of radial diffusion, so long as the requirements ofhydrodynamic stability are fulfilled. Hydrodynamic stability requiresthat the separate gas streams be ordered radially according to the massdensity of their constituents, with the lightest gas innermost (the onewith the lowest average atomic or molecular weight) and the heaviest gasoutermost. This condition is expressed mathematically as d/dr (lnρ)>−d/dr (ln T), where ρ is the fluid mass density and T is the plasmatemperature. If the layered rotating system does not satisfy this rule,an hydrodynamic instability will cause radial convection which rapidlymixes the concentric gas streams.

Referring to the embodiment shown in FIG. 17, a first gas 173 of lowmass density is injected through an axial tube 174 which co-rotates withthe envelope 10 and the end cap 44. To prevent radial spreading of theaxial stream over the axial distance between the tube mouth and thebeginning of the discharge column, it is convenient to make the feedtube 174 an electrode of the arc discharge. A second gas 175 of highermass density is admitted through collimating holes 50 in the end cap 44.Passage of the first gas 173 through the small bore of the axial tube174 transfers rotational momentum to it; passage of the second gas 175through the collimating holes in end cap 44 does the same for it. Boththe cup 48 and the axial feed tube 173 passing through it, make slidingseals against the end cap 44. Because the axial tube 174 lies whollyinside the radius of the collimating holes 50, the first gas 173 formsan axial stream 173A which is surrounded by an annular stream 175A ofthe second gas 175.

At the interface between the axial and annular streams 173A and 175A,respectively, mixing occurs only by the slow process of diffusionproviding that the gases in the two streams are ordered by mass densityas previously stated and the relative axial velocity of the two streamsis sufficiently low for laminar flow to obtain at their boundary. If theinner and outer streams contain a fuel and an oxidant, an exothermicchemical reaction (i.e., combustion) may occur at their interface. Thisreaction can heat and/or sustain a plasma column if the interface liesnear or inside the critical radius r_(c) for the rotation speed of theenvelope 10.

In two-stream systems heated by non-chemical means, xenon is a preferredchoice of gas for the annular stream because of its low thermalconductivity (roughly ⅕ that of air) and its relatively high atomicweight. A sheath of xenon surrounding a central plasma column formed ina gas of lower mass density provides excellent thermal insulation. Argonis an attractive gas for use in the axial stream, because of the easewith which it can be ionized.

Another use of the axial gas stream 173A is to inject entrained samples,such as small droplets of a salt solution created with a nebulizer, intothe plasma column. Following atomization in the hot plasma column, theconstituent elements of these salts and their solvent will remainconcentrated within the plasma column, providing the fluid mass densityin the plasma column is less than that of the surrounding annularstream. In atomic emission spectroscopy, this entrainment method iscommonly used to feed samples into atmospheric pressure, arc sourceswhich excite the characteristic spectral lines of these elements.Conventional arc source permit single-pass excitation of such samples,whereas the concentrating properties of the present invention allow forretention of the dopant over an extended axial path and, therefore,greater sensitivity for its detection.

Referring now to FIG. 18, shown is a side cross-sectional view of adiffusive separator based on the dual gas embodiment of FIG. 17. Adiffusive separator, as illustrated in FIG. 18, may be constructed basedon the principles of the dual stream flow described above and shown inFIG. 17. The structure of the diffusive separator is very similar to thedual gas embodiment of FIG. 17, except the containment envelope 10 ofthe diffusive separator based on a rotating discharge tube has an innertube 200 and an outer tube 201. The outer tube 201 is stationary and isoutfitted along its length with ports 204 for extraction ofmass-separated material. The inner tube 200 rotates (as indicated byarrow 210) and is comprised of a stack of hollow disks 206 separatedfrom one another by small gaps 208 and connected by a plurality of rods212 preferably made from or covered by a non-conductive, heat resistantmaterial such as a ceramic. The axial spacing 208 of disks 206 is chosenso that the inner tube 200 formed by this assemblage of disks 206permits waste atoms to diffuse radially to the wall of the outerstationary tube 201, while still maintaining a rigid rotor flow of gasinside the inner tube 200.

Material 175 introduced from the left into the diffusive separator, isatomized in the hot plasma column 21. If originally molecular in nature,the material 175 will be dissociated into its elemental constituents.The cold gas surrounding the hot plasma column 21 is the medium throughwhich the hazardous waste material diffuses, once that material 175 hasbeen atomized. Since the diffusion rate for radial migration of wasteatoms through the surrounding gas is proportional to their thermalspeed, light-weight atoms diffuse more rapidly than heavy ones. With aslow axial flow of the waste material through the discharge region,i.e., the central plasma column 21, a mass separation of the waste atomsis accomplished.

In the diffusive separator depicted in FIG. 18, the combined processesof molecular dissociation in the hot plasma column 21 followed by massseparation of the constituent atoms may be utilized to process mixedwaste into radioactive and non-radioactive components. Becauseradioactive elements are, for the most part, heavy elements, aseparation by mass enables the extraction of an axial beam in which theradioactive component of the waste is enriched. The degree of enrichmentmay be increased by repetitive cycling of the axial output through thesystem.

FIG. 19 is a side cross sectional view of an embodiment employing amedium pressure mercury lamp.

Shown is a cylindrical quartz envelope 300 with hemispherical ends 302enclosing a discharge region between two electrodes 304. Each electrode304, comprises a tungsten rod 306 over-wound with thoriated tungstenwire 308 in the form of a coil, is connected electrically to theexternal metal cap 310 by a molybdenum foil 312 to which the necked-downend of the quartz envelope makes a hermetic seal. Reflective coatings314 on each of the hemispherical ends 302 of the quartz envelope 300reflects infrared light back through the quartz envelope, insuring thata region behind tips 316 of the electrodes 304 does not act as a coldsink for condensation of mercury from a fill gas 318 within the quartzenvelope 300. Secured by a nut 320 that is threaded onto each of themetal end caps, rotors 322 at opposite ends of the medium pressuremercury lamp rotate inside air bearings 324 attached to a mechanicalsupport structure 326. Electrical connection of a power supply (notshown) to the medium pressure mercury lamp is achieved by means ofelectrical brushes 328. The structure and methodologies associate withthe medium pressure mercury discharge lamp are well known, and thereforefurther description thereof is not made herein except to the extent suchstructures and methodologies have been altered for purposes of thepresent embodiment.

Advantageously, the electrical brushes 328, while providing the currentpath, allow rotation of the rotors 322 relative to the electricalbrushes 328. Use of electrical brushes 328 to allow rotation of a rotor322 in this way in known, e.g., in the field of electrical motors, andtherefore further description of the structure and methodologiesassociated with the brushes is not made herein.

Cylindrical air bushings that form the air bearings 324 contain a porousliner through which pressurized gas flows to form a film of air betweena close-fitting shaft of the rotor 324 that rotates inside it. Since thedimensional tolerances on the shaft diameter are very tight(+0.0002/−0.0000 inches), clean air must be supplied to the airbearings. For this purpose, special filters are employed to eliminateparticulates and oil. Air bearings have the desirable features ofproviding nearly frictionless motion and long operational life. However,conventional rolling bearings or magnetic bearings are also candidatesfor use in a rotating tube discharge containing a medium pressuremercury lamp.

In order to impart a rotational force to the rotors 322, respectiveflanges 326 are employed at respective edges of the rotors 322 proximateto the metal end caps 310. The flanges 326 are positioned againstrespective edges 328 of the air bearings 324, with the small tolerancesbeing employed between the flanges 326 and the edges.

Referring to FIG. 20, an end view of rotor 322 is shown. The flanges 326are shown having turbine grooves 400 on respective faces 402 of theflanges 326 proximate to the respective edges 328 of the of the airbearings 324.

Referring back to FIG. 19 and continuing to refer to FIG. 20, theturbine grooves 400 provide relatively low resistance paths for the airto move between the flanges 326 and the edges 328. As a result of thislow resistance, a greater velocity and lower pressure air flow iscreated within the turbine grooves 400, thereby creating an increasedvolume of air flow within the turbine grooves 400 relative to the airflow over the remainder of the faces 402 within the small tolerancesbetween the flanges 326 and the edges 328. Because the turbine grooves400 are curved in a consistent direction, the turbine groves 400 bendwhat would otherwise be radial air flow into air flow that includes atangential component as well. This tangential component imparts arotational force (moment) to the flanges 326 of the rotors 322 as theair moves out of the turbine groves, thereby causing the rotors 322 torotate. Because the air bearings provide a very low frictionrelationship between the rotors 322 and the air bearings 324, rotationof the rotors 322 in this manner is highly efficient. Alternate means ofrotation may be used, such as an in-line electric motor in which anaxial extension of the medium pressure mercury lamp forms the armatureof the in-line electric motor. Belt-driven or gear driven rotation froman external motor is also possible.

A commercial medium pressure lamp with a two inch nominal arc length(Jelight model J02PM2HGC1) was operated in “rotating tube discharge”(RTD) mode. The operating parameters recommended for this lamp by themanufacturer are 100 volts at 4.3 amperes. With a rotational speed ofapproximately 1500 rpm, a stable arc could be maintained in the lampover more than an order of magnitude range in current. When the currentwas lowered to 0.4 amperes, the heating of the quartz envelope wasinsufficient to maintain the entire charge of mercury in vapor form and,consequently, the measured ultraviolet emission decreased with timeuntil the pressure stabilized at a new level corresponding to the vaporpressure of mercury at the lower, steady-state wall temperature.

FIG. 21 is a graph showing the cross-sectional area of the plasma columnversus current at constant pressure for the medium pressure mercury lampdescribed above. (A separate scale for the diameter of the plasma columnis shown on the righthand ordinate.) The cross sectional area scalesroughly linearly with current in this radiation-dominated arc. At thelowest current, i.e., approximately 0.4 amperes, the plasma column isonly three millimeters in diameter. By comparison, the inside diameterof the quartz envelope is twenty-one millimeters. With a thick annularlayer of neutral gas separating the plasma column from the wall, thethermally conducted heat loss to the wall of the quartz envelope inrotating discharge mode is significantly reduced compared to that innormal wall-stabilized operating mode.

FIG. 22 is a graph showing relative UV radiation efficiency versuscurrent in the present embodiment for the two inch medium pressuremercury lamp. Here, the radiative efficiency plotted is the measuredintensity of the Hg I 365 nm line divided by the inter-electrodal ohmicpower dissipation per inch. However, the latter quantity can only beestimated, since total gap voltage is the quantity that is measured.Using the manufacturer's voltage specifications for long medium pressuremercury lamps with a recommended operating current of 4.0 amperes, anelectrodal drop of 110 volts is inferred. This is roughly 85% of themeasured gap voltage in the two inch medium pressure mercury lamp.

Because of the uncertainty in the denominator of the radiativeefficiency, dashed curves are drawn above and below the measured datapoints, indicating likely errors bars in the plotted data. The datasuggests that a peak in radiative efficiency is achieved at currents inthe range 25%-50% of that recommended by the manufacturer.

Operation of medium pressure mercury lamps in rotating tube discharge(RTD) mode at a current less than that required for conventionalwall-stabilized operation confers several benefits. First, improvedradiative efficiency at reduced current translates into electricalsavings, particularly for long medium pressure mercury lamps with highlinear power ratings. Second, the reduced temperature of the electrodesand of the quartz envelope promise a longer bulb lifetime, because theprocesses leading to gradual degradation in ultraviolet transmission arestrongly temperature-dependent. Thirdly, rotating tube dischargeoperation completely suppresses the snaking phenomenon observed inmedium pressure mercury tubes of intermediate length at low current orwith the presence of metal additives.

The rotating tube discharge method of operation may be applied to othercommercial lamps. Xenon flashlamps have fill pressures in the relevantregime for successful rotating tube discharge operation. High intensitydischarge (HID) lamps may also be operated in a rotating tube dischargemode. By freeing the lamp designer from the requirement that the arc runin an electrode-stabilized or wall-stabilized mode, rotating tubedischarge operation permits a more flexible use of metal additives inshaping the spectral distribution of the visible output. Rotating tubedischarge operation may be applied to all standard high intensitydischarge lamps, such as sodium, mercury and metal halide high intensitydischarge lamps. In addition, if electrodeless heating methods areemployed, it may used with fills like sulfur to create long, straightplasmas columns which emit a solar-like spectrum in the visible.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. An apparatus for forming a stable equilibrium for a high pressureplasma column comprising: i) a containment envelope having aplasma-forming fill inside, ii) means for rotating the containmentenvelope about a horizontal, or substantially horizontal axis, iii)means for initiating a discharge in said plasma-forming fill, iv) meansfor heating and sustaining a resulting plasma in a steady state orpulsed manner; and v) means for extending a length of the resultingplasma.
 2. The apparatus as in claim 1, wherein said containmentenvelope is of any shape as long as its minimum cross sectionaldimension perpendicular to its rotation axis exceeds a radius at which acentrifugal force, for a chosen rotation speed, dominates overgravitational force.
 3. The apparatus as in claim 2, wherein said meansfor initiating a discharge in said plasma-forming fill comprises atleast one of electrical, electromagnetic or and chemical sources ofenergy.
 4. The apparatus as in claim 3, wherein said means for heatingand sustaining the resulting plasma comprises at least one ofelectrical, electromagnetic or chemical sources of energy, individuallyor in combination with one another.
 5. The apparatus as in claim 4,wherein said containment envelope is sealed so as to permit operation atat least one of sub-atmospheric, atmospheric and super-atmosphericpressures with plasma-forming fills comprised of material that undernormal conditions is gaseous, or that changes from solid or liquid forinto gaseous phase during operation of the apparatus.
 6. The apparatusas in claim 5, wherein said “containment envelope having aplasma-forming fill inside” includes a medium-pressure, mercury-vaporare lamp or a xenon flashlamp, of a standard commercial variety or onewith an envelope customized to facilitate rotational operation.
 7. Theapparatus as in claim 5, where said “containment envelope having aplasma-forming fill inside” comprises high intensity discharge (HID)lamps for general illumination which, in operation, contain highpressure vapors of at least one of sodium, and mercury.
 8. The apparatusas in claim 5, wherein said means for heating and sustaining the plasmacolumn repeats in modular form along the rotation axis so as to producethe plasma column many times longer in an axial direction than oneheated at a single location only.
 9. The apparatus as in claim 5 for useas a light source, wherein a window is mounted at one end of thecontainment envelope to permit transmission of an ultraviolet, visible,or infrared region of a spectrum of a light beam in the direction of therotation axis.
 10. The apparatus as in claim 9 for use as a source ofemission spectra, wherein an existence of a unique plasma temperatureand that of an axially uniform electron density and temperature permit adetermination of atomic constants based on a Boltzwellian populationdistribution among excited states.
 11. The apparatus as in claim 5 foruse as an X-ray source, wherein a window is mounted at one end of thecontainment envelope to permit transmission of X-ray line radiationcharacteristic of each element (K-alpha radiation) and/or X-raycontinuum radiation.
 12. An apparatus for forming a stable equilibriumfor a high pressure plasma column comprising: i) a containment envelopehaving a plasma-forming fill inside, ii) means for rotating thecontainment envelope about a horizontal, or substantially horizontalaxis, iii) means for initiating a discharge in said plasma-forming fill,and iv) means for heating and sustaining a resulting plasma in a steadystate or pulsed manner; wherein said containment envelope is open-endedso as to permit a flow of gases along its rotation axis.
 13. Theapparatus as in claim 12, wherein said means for heating and sustainingthe plasma column repeats in modular form along the rotation axis so asto produce the plasma column many times longer in an axial directionthan one heated at a single location only.
 14. The apparatus as in claim12 or 13, wherein the apparatus further comprises: means for ensuringthat the gases flowing through said envelope co-rotate with the envelopein close approximation to rigid rotor motion.
 15. The apparatus as inclaim 14, wherein said plasma column is elongated in the direction ofthe rotation axis to produce a plume or “flame” that extends beyond theopen end of the containment envelope.
 16. The apparatus as in claim 14,wherein two concentric gas streams flowing laminarly through saidenvelope mix with one another only by a slow process of radial diffusionif those streams are arranged in a radially ordered system such thatmass densities (specific weights) of the gas streams increase withradius.
 17. The apparatus as in claim 16, wherein dopants introduced inan axial stream of the two concentric gas streams of said radiallyordered system remain concentrated in the axial stream if the massdensity of the gas in the axial stream (following atomization of thedopant-containing substance) is less than that the gas in an annularstream of the two concentric gas streams immediately surrounding theaxial stream.
 18. The apparatus as in claim 17, wherein materialintroduced into the axial gas stream is dissociated in the hot plasmacolumn into its elemental constituents and the resulting atoms diffusethrough the surrounding mantle of co-moving neutral gas with speedsvarying inversely with mass such that a separation by mass of theelemental constituents may be realized.
 19. The apparatus as in claim18, wherein mixed waste introduced in the axial gas stream is separatedby mass such that the axial stream becomes enriched in radioactivecomponent which, in general, comprises heavier elements thannon-radioactive chemical component of the mixed waste.
 20. The apparatusas in claim 12 for use as a plasma torch, wherein a plasma flame whichextends beyond an end of the containment envelope is sustained byelectrodeless means or by electric current between hollow electrodes.21. The apparatus as in claim 20 for use as a source of emissionspectra, wherein an existence of a unique plasma temperature and aneasily measurable electron density permits a determination of atomicconstants, assuming a Boltzwellian population distribution among excitedstates.
 22. The apparatus as in claim 12 for use in chemical processingof pure gas streams and gas streams with entrained liquids and solids,wherein the plasma column substantially fills a bore of the containmentenvelope.
 23. An apparatus for forming a high pressure plasma columncomprising: i) a containment envelope having a plasma-forming fillinside, ii) means for rotating the containment envelope about an axis,iii) means for initiating a discharge in said plasma-forming fill, andiv) means for heating and sustaining a resulting plasma in a steadystate or pulsed manner comprising at least one of electrical lightwaves, and chemical sources of energy.
 24. The apparatus as in claim 23,wherein said containment envelope is of any shape as long as its minimumcross sectional dimension perpendicular to its rotation axis exceeds aradius at which a centrifugal force, for a chosen rotation speed,dominates over gravitational force.
 25. The apparatus as in claim 24,wherein means for initiating a discharge in said plasma-forming fillincludes electrical, electromagnetic or chemical sources of energy. 26.The apparatus as in claim 23, wherein said containment envelope issealed so as to permit operation at sub-atmospheric, atmospheric orsuper-atmospheric pressure with plasma-forming fills comprised ofmaterial that under normal conditions is gaseous, or that is changedfrom solid or liquid form into gaseous phase during operation of theapparatus.
 27. The apparatus as in claim 26, wherein said “containmentenvelope having a plasma-forming fill inside” comprises at least one ofa medium-pressure, mercury-vapor arc lamp and a xenon flashlamp.
 28. Theapparatus as in claim 26, where said “containment envelope having aplasma-forming fill inside” comprise high intensity discharge (HID)lamps for general illumination which, in operation, contain highpressure vapors of at least one of sodium, and mercury.
 29. Theapparatus as in claim 27, wherein said means for heating and sustainingthe plasma column repeats in modular form along the rotation axis so asto produce the plasma column many times longer in an axial directionthan one healed at a single location only.
 30. An apparatus for forminga high pressure plasma column comprising: i) a containment envelopehaving a plasma-forming fill inside, ii) means for rotating thecontainment envelope about an axis, iii) means for initiating adischarge in said plasma-forming fill, and iv) means for heating andsustaining a resulting plasma in a steady state or pulsed manner;wherein said containment envelope is open-ended so as to permit a flowof gases along its rotation axis.
 31. The apparatus as in claim 30,wherein said means for heating and sustaining the plasma column repeatsin modular form along the rotation axis so as to produce the plasmacolumn many times longer in an axial direction than one heated at asingle location only.
 32. The apparatus as in claim 30 or 31, furthercomprising means for ensuring that the gases flowing through saidenvelope co-rotate with the envelope in close approximation to rigidrotor motion.
 33. The apparatus as in claim 32, wherein said plasmacolumn is elongated in the direction of the rotation axis to produce aplume or “flame” that extends beyond the open end of the containmentenvelope.
 34. The apparatus as in claim 32, wherein two concentric gasstreams flowing laminarily through said envelope mix with one anotheronly by a slow process of radial diffusion if those streams are arrangedin a radially ordered system such that mass densities (specific weights)of the gas streams increase with radius.
 35. The apparatus as in claim34, wherein dopants introduced in an axial stream of the two concentricgas streams of said radially ordered system remain confined in the axialstream if the mass density of the gas in the axial stream is less thanthat of the gas in an annular stream of the two concentric gas streamsimmediately surrounding the axial stream.
 36. The apparatus as in claim30 for use as a plasma torch, wherein a plasma flame which extendsbeyond an end of the containment envelope is sustained by electrodelessmeans or by electric current between hollow electrodes.